Polymeric materials of the styrene family including polystyrene and its copolymers, impact resistant polystyrene containing dispersed rubber phases as polyblends and more recently impact resistant copolymers and terpolymers of styrene have gained great commercial utility as tough engineering plastics. Such plastics appear as structural parts of appliances, automobiles and housing.
These structural parts may be metal plated for abrasion and corrosion resistance and for decorative finishes. The polymeric parts are lightweight, hence, the metal plated polymeric parts are more desirable than solid metal parts for many uses. It is recognized that metal plated plastics have some resistance to fire, however, the Underwriters Laboratory do not classify plated materials as fire-retarding or self-extinguishing unless the plastic itself is inherently self-extinguishing. If the plating fails or is not continuous the plastic article will burn. Further, when the part melts and the plastic breaks through the plating it will support combustion unless the plastic is self-extinguishing.
Many such plastic articles are not considered metal platable because the metal plating will not adhere to the plastic substrate. Stringent adhesion tests are required along with thermal cycling tests to determine if the plating is a functional plating and will not crack, blister or craze during use because of poor adhesion, It is the objective of this invention to provide particular polymeric articles of the styrene family which are metal platable and self-extinguishing providing great utility.
Government codes are requiring that such plastics be flame-retardant. The industry has developed may improved grades of such materials, however, with the ever greater need for high performance plastics, most fire retardant polymers have suffered from inadequate physical properties such as toughness and impact strength.
Polymers formulated with various additives in sufficient amounts to produce fire-retardant properties in the polymer may cause certain physical properties of the polymer to be seriously impaired. Flame-proofing materials such as the halogenated aliphatic and aromatic compounds which are compatible with the styrene polymer family of plastics often act as plasticizers and lower the modulus, reducing the tensile strength. Such materials are thought to decompose and provide a halogen gas-phase that inhibits the gas-phase combustion of the plastic.
Certain inorganic compounds, in particular metal oxide compounds when used in combination with the halogenated organic compound, appear to catalyze their decomposition or enter the chain of reactions to form metal halides that are effective flame retardants. Such system are not predictable in that many such combinations lower the melt point of the polymer causing it to pyrolyze more readily, hence, actually increasing flammability. Beyond flammability such combinations have caused the polymer systems to degrade during heat processing or on exposure to light.
Polymeric plasticizers containing halogens such as polyvinyl chloride and chlorinated olefins have been tried in an attempt to overcome the deficiencies of the lower molecular weight halogenated organic compounds. Such polymeric materials, however, as used in the styrene family of plastics have lowered their heat stability during processing and given lower physical properties particularly lower modulus, heat distortion and impact strength.
The above problems have been overcome in the styrene family of impact polymeric polyblends by the present invention of incorporating a particular grafted chloroprene rubber that will provide both self-extinguishing properties and metal platability along with impact strength and toughness. It has been further discovered that the chloroprene rubber when used in combination with certain metal oxides in the polyblends gives even further improvement in self-extinguishing properties.